Field of the Invention
The present invention relates to a positive electrode active material having improved safety and lifetime characteristics and a secondary battery comprising the same.
Description of the Related Art
Recently, in line with growing concerns about environmental issues, a significant amount of research into electric vehicles and hybrid electric vehicles capable of replacing vehicles using fossil fuels such as gasoline vehicles and diesel vehicles, one of major causes of air pollution, has been conducted. Although nickel-metal hydride secondary batteries have been mainly used as a power source of the electric vehicles and hybrid electric vehicles, research into using lithium secondary batteries having high energy density and discharge voltage as well as long cycle lifetime and low self-discharge rate has been actively conducted and some of the research are in a commercialization stage.
A carbon material is mainly used as a negative electrode active material of the lithium secondary batteries and the use of lithium or a sulfur compound is also considered. Also, lithium-containing cobalt oxide (LiCoO2) is mainly used as a positive electrode active material and in addition, the use of lithium-containing manganese oxides, such as LiMnO2 having a layered crystal structure or LiMn2O4 having a spinel crystal structure, and lithium-containing nickel oxide (LiNiO2) is also considered.
LiCoO2 among the positive electrode active materials is most widely used due to its excellent lifetime characteristics and charge-discharge efficiency. However, LiCoO2 has limitations in massively being used as a power source for an industrial sector such as electric vehicles, because structural stability thereof may decrease and price competitiveness may be limited due to its high price according to the resource limitation of cobalt used as a raw material.
A LiNiO2-based positive electrode active material is relative inexpensive and exhibits battery characteristics having high discharge capacity. However, rapid phase transition of its crystal structure may occur according to volume changes accompanied by a charge and discharge cycle, and stability may rapidly decrease when being exposed to air and moisture.
In contrast, since lithium manganese oxide has advantages in that resource thereof as a raw material is abundant and environmentally friendly manganese is used, lithium manganese oxide attracts great interests as a positive electrode active material capable of replacing LiCoO2. In particular, spinel-structured lithium-containing manganese oxide has advantages in that thermal stability may be excellent, the price may be low, and synthesis may be facilitated. However, the spinel-structured lithium-containing manganese oxide has disadvantages in that capacity may be low, lifetime characteristics may be decreased due to a side reaction, and cycle characteristics and high-temperature storage characteristics may be poor.
As a result, layer-structured lithium-containing manganese oxide is suggested in order to compensate for the low capacity of spinel and secure excellent thermal safety of manganese-based active materials. In particular, layer-structured xLi2MnO3(1−x)LiMO2 (0<x<1, M=Co, Ni, Mn, etc.) having a content of manganese (Mn) greater than those of other transition metal(s) has disadvantages in that initial irreversible capacity thereof may be relatively high. However, since relatively high capacity may be manifested during initial charge at a high voltage, layer-structured xLi2MnO3(1−x)LiMO2 becomes a subject of active research as a positive electrode active material.
In the case that charging is performed at a relatively high voltage of 4.5 V (specifically, 4.4 V or more) based on an initial positive electrode potential, the lithium-containing manganese oxide exhibits high capacity reaching about 250 mAh/g as well as an excessive amount of gas, such as oxygen and carbon dioxide, being generated, while exhibiting a plateau potential range of 4.4 V to 4.8 V.
The plateau potential range and the generation of gas in characteristic features relating to structural changes in the material may also continuously occur in a subsequent cycle in the case that the plateau potential range and the generation of gas do not sufficiently occur during initial charge, i.e., a formation process.
Therefore, in order to obtain such a high capacity as above by using the layer-structured xLi2MnO3(1−x)LiMO2 as a positive electrode active material, it may be essential to sufficiently perform a formation process at a voltage of 4.4 V or more based on the positive electrode potential as described above. However, since a side reaction, such as oxidation of an electrolyte, may be facilitated at a voltage of the plateau potential or more, conditions, such as formation voltage and time, may be inevitably limited. Therefore, since the initial formation is insufficiently performed, gas composed of oxygen and carbon dioxide is continuously released during a repetitive charge and discharge cycle, and thus, safety and lifetime characteristics of the battery may be decreased.
Accordingly, in Korean Patent Application Laid-Open Publication Nos. 10-2007-0012213 and 10-2007-0021955, methods of charging and discharging are disclosed, in which a formation process is performed at a high voltage during a first cycle, gas is then removed through a degassing process, and thereafter, a voltage is decreased to a level of an operating voltage of a general secondary battery (4.4 V or less). However, a phenomenon of having insufficient formation performed during initial charge is the same even by the foregoing methods, and as a result, gas is continuously generated during the use of the battery and low capacity is obtained in comparison to the case of performing a cycle at a high voltage of 4.5 V or more. Therefore, there is a continuous need for research into a technique for addressing the foregoing limitations.
Thus, with respect to a lithium secondary battery comprising the layer-structured lithium-containing manganese oxide xLi2MnO3(1−x)LiMO2 as a positive electrode active material, there is a need to develop a technique for addressing the continuous generation of a large amount of gas during a cycle at a high voltage.